Carbon source/sink function of a subtropical, eutrophic lake determined from an overall mass balance and a gas exchange and carbon burial balance

Although studies on carbon burial in lake sediments have shown that lakes are disproportionately important carbon sinks, many studies on gaseous carbon exchange across the water-air interface have demonstrated that lakes are supersaturated with CO(2) and CH(4) causing a net release of CO(2) and CH(4) to the atmosphere. In order to more accurately estimate the net carbon source/sink function of lake ecosystems, a more comprehensive carbon budget is needed, especially for gaseous carbon exchange across the water-air interface. Using two methods, overall mass balance and gas exchange and carbon burial balance, we assessed the carbon source/sink function of Lake Donghu, a subtropical, eutrophic lake, from April 2003 to March 2004. With the overall mass balance calculations, total carbon input was 14 905 t, total carbon output was 4950 t, and net carbon budget was +9955 t, suggesting that Lake Donghu was a great carbon sink. For the gas exchange and carbon burial balance, gaseous carbon (CO(2) and CH(4)) emission across the water-air interface totaled 752 t while carbon burial in the lake sediment was 9477 t. The ratio of carbon emission into the atmosphere to carbon burial into the sediment was only 0.08. This low ratio indicates that Lake Donghu is a great carbon sink. Results showed good agreement between the two methods with both showing Lake Donghu to be a great carbon sink. This results from the high primary production of Lake Donghu, substantive allochthonous carbon inputs and intensive anthropogenic activity. Gaseous carbon emission accounted for about 15% of the total carbon output, indicating that the total output would be underestimated without including gaseous carbon exchange.     

Pools and fluxes of organic matter in a boreal landscape: implications for a safety assessment of a repository for nuclear waste

To provide information necessary for a license application for a deep repository for spent nuclear fuel, the Swedish Nuclear Fuel and Waste Management Co is carrying out site investigations, including extensive studies of different parts of the surface ecosystems, at two sites in Sweden. Here we use the output from detailed modeling of the carbon dynamics in the terrestrial, limnic and marine ecosystems to describe and compare major pools and fluxes of organic matter in the Simpevarp area, situated on the southeast coast of Sweden. In this study, organic carbon is used as a proxy for radionuclides incorporated into organic matter. The results show that the largest incorporation of carbon into living tissue occurs in terrestrial catchments. Carbon is accumulated in soil or sediments in all ecosystems, but the carbon pool reaches the highest values in shallow near-land marine basins. The marine basins, especially the outer basins, are dominated by large horizontal water fluxes that transport carbon and any associated contaminants into the Baltic Sea. The results suggest that the near-land shallow marine basins have to be regarded as focal points for accumulation of radionuclides in the Simpevarp area, as they receive a comparatively large amount of carbon as discharge from terrestrial catchments, having a high NPP and a high detrital accumulation in sediments. These focal points may constitute a potential risk for exposure to humans in a future landscape as, due to post-glacial land uplift, previous accumulation bottoms are likely to be used for future agricultural purposes.     

Relationships among trihalomethane formation potential, organic carbon and lake enrichment

Trihalomethanes (THMs) are potential carcinogens formed from the reaction of the disinfectant chlorine with organic matter in the source water. This study of Kansas drinking water supply lakes evaluates the relationship among THM formation potential (THMFP), organic carbon and lake trophic state (LTS). THMFP was positively correlated to organic carbon. Total THMFP and total organic carbon were positively correlated to LTS, an estimator of lake enrichment, when very turbid lakes were omitted. These very turbid lakes (due to high suspended solids concentrations) had higher than expected THMFP, based on LTS, and higher organic carbon concentrations. THM data measured in the treated drinking water were positively correlated to THMFP, total organic carbon and LTS. The levels of organic carbon that contribute to THMs are a result of lake and watershed factors related to increasing levels of enrichment and suspended sediments. These factors are controllable by appropriate management practices.     

Comparison of a mass balance and an ecosystem model approach when evaluating the carbon cycling in a lake ecosystem

Carbon budgets are frequently used in order to understand the pathways of organic matter-in ecosystems, and they also have an important function in the risk assessment of harmful substances. We compared two approaches, mass balance calculations and an ecosystem budget, to describe carbon processing in a shallow, oligotrophic hardwater lake. Both approaches come to the same main conclusion, namely that the lake is a net autotrophic ecosystem, in spite of its high dissolved organic carbon and low total phosphorus concentrations. However, there were several differences between the carbon budgets, e.g. in the rate of sedimentation and the air-water flux of CO2. The largest uncertainty in the mass balance is the contribution of emergent macrophytes to the carbon cycling of the lake, while the ecosystem budget is very sensitive towards the choice of conversion factors and literature values. While the mass balance calculations produced more robust results, the ecosystem budget gave valuable insights into the pathways of organic matter transfer in the ecosystem. We recommend that when using an ecosystem budget for the risk assessment of harmful substances, mass balance calculations should be performed in parallel in order to increase the robustness of the conclusions.     

Use of O2 consumption and CO2 production in kinetic cells to delineate pyrite oxidation-carbonate buffering and microbial respiration in unsaturated media

Identifying zones of sulphide oxidation and carbonate buffering is important in the development of a management plan for mine waste-rock piles. In this study, we used a kinetic cell technique to measure rates of O2 consumption and CO2 production in low sulphide (<0.12 wt.% S), low inorganic carbon (<0.20 wt.% C(inorganic)), gneissic waste rock and associated organic-rich lake sediment (0.7 wt.% C(organic)), and forest soil (1.4 wt.% C(organic)) collected from the Key Lake uranium mine in Saskatchewan, Canada. Solid chemistry, stable carbon isotope, pore water sulphate concentration data, and stoichiometric considerations indicated that O2 consumption and CO2 production were constrained by microbial respiration in the lake sediment and forest soil and by pyrite oxidation-carbonate buffering in the gneissic waste rock. Mean ratios of molar CO2 production to O2 consumption rates were 0.5 for lake sediment, 0.7 for forest soil, and 0.2 for gneissic waste rock. The different O2/CO2 ratios suggested that O2-CO2 monitoring may provide a practical tool for identifying the zones of microbial respiration and pyrite oxidation-carbonate buffering in mine waste-rock piles. Rates of O2 consumption and CO2 production were about one order of magnitude greater in lake sediment than in gneissic waste rock, indicating that microbial respiration would exert a control on the distribution of O2 and CO2 gas in waste-rock piles constructed upon the dewatered lake sediments.     

Modelling geochemical and microbial consumption of dissolved oxygen after backfilling a high level radiactive waste repository

Dissolved oxygen (DO) left in the voids of buffer and backfill materials of a deep geological high level radioactive waste (HLW) repository could cause canister corrosion. Available data from laboratory and in situ experiments indicate that microbes play a substantial role in controlling redox conditions near a HLW repository. This paper presents the application of a coupled hydro-bio-geochemical model to evaluate geochemical and microbial consumption of DO in bentonite porewater after backfilling of a HLW repository designed according to the Swedish reference concept. In addition to geochemical reactions, the model accounts for dissolved organic carbon (DOC) respiration and methane oxidation. Parameters for microbial processes were derived from calibration of the REX in situ experiment carried out at the Asp? underground laboratory. The role of geochemical and microbial processes in consuming DO is evaluated for several scenarios. Numerical results show that both geochemical and microbial processes are relevant for DO consumption. However, the time needed to consume the DO trapped in the bentonite buffer decreases dramatically from several hundreds of years when only geochemical processes are considered to a few weeks when both geochemical reactions and microbially-mediated DOC respiration and methane oxidation are taken into account simultaneously.     

Influence of organic matter on municipal solid waste incinerator bottom ash carbonation

The biodegradation of organic matter in municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the interaction between the CO(2) produced by microbial respiration and bottom ash. Respiration tests were performed on bottom ash at different moisture contents in an incubator at 30 degrees C. O(2) consumption and CO(2) production were monitored and quantified. Leaching tests were carried out at the end of the experiments. Total organic carbon (TOC) leaching had decreased. Over a period of three weeks, pH decreased from 10.7 to 8.2 and bottom ash was considered to be fully carbonated. This showed that the organic matter found in bottom ash can provide a substrate for microbial activity. The CO(2) produced by microbial respiration was directly dissolved in bottom ash pore water in order to be mineralized in carbonate form. The origin of the carbon dioxide which induces maturation of bottom ash on weathering areas has never been really discussed and is often presumed to be atmospheric CO(2). However, biodegradation of organic matter could contribute for a large part to this phenomenon, depending on field-scale physico-chemical weathering conditions.     

Carbon exchange between ecosystems and atmosphere in the Czech Republic is affected by climate factors

By comparing five ecosystem types in the Czech Republic over several years, we recorded the highest carbon sequestration potential in an evergreen Norway spruce forest (100%) and an agroecosystem (65%), followed by European beech forest (25%) and a wetland ecosystem (20%). Because of a massive ecosystem respiration, the final carbon gain of the grassland was negative. Climate was shown to be an important factor of carbon uptake by ecosystems: by varying the growing season length (a 22-d longer season in 2005 than in 2007 increased carbon sink by 13%) or by the effect of short- term synoptic situations (e.g. summer hot and dry days reduced net carbon storage by 58% relative to hot and wet days). Carbon uptake is strongly affected by the ontogeny and a production strategy which is demonstrated by the comparison of seasonal course of carbon uptake between coniferous (Norway spruce) and deciduous (European beech) stands.     

Volatile organic sulfur compounds in a stratified lake

Three volatile organic sulfur compounds (VOSCs), dimethyl sulfide (DMS), carbon disulfide (CS(2)), and dimethyl disulfide (DMDS), were detected in the stratified water column of a lake (Linsley Pond) in Connecticut. The compounds DMS and DMDS appeared in both the oxic and the anoxic portions of the water column, CS(2) was primarily found in anoxic hypolimnion. Algal metabolism and/or bacterial degradation of sulfur-containing amino acids or other organic materials are potential sources of VOSCs in the oxic lake water. Reactions of hydrogen sulfide with organic compounds and microbial degradation of organic matter may be responsible for the production of VOSCs in the anoxic lake water. The vertical distribution patterns of these three VOSCs varied from month to month in the summer, but the daily profiles obtained in one 5-day period in the summer displayed consistency. No clear diurnal pattern for any of the three VOSCs was observed. Based on observation that these VOSCs were not present in surface and near surface waters of Linsley Pond, freshwater inputs of reduced sulfur compounds to the atmosphere may be insignificant.     

UV-light induced mineralization of organic matter bound chlorine in Lake Bjän,Sweden--a laboratory study

Surface water and aqueous solutions of isolated organic matter from a humic rich lake in southern Sweden were exposed to artificial UV radiation to investigate the UV light induced influence on organic matter bound chlorine in natural systems. It was found that the photodegradation of organic matter bound chlorine was more pronounced than the photodegradation of organic carbon. After 120 h of irradiation of the isolated organic matter, only 35% of the initial organochlorine was still in the solution compared to about 70% of the dissolved organic carbon (DOC). A similar result was obtained for unfractionated surface water. Furthermore, our results indicate that the loss of organic chlorine was mainly due to a mineralization of organic chlorine into chloride ions. The total decrease of organic chlorine after 120 h was 32 microg Cl(org) l(-1), of which the major part disappeared in the initial irradiation phase. A similar increase was observed in the chloride concentration (34 microg Cl(-) l(-1)).     

Influence of biowaste compost amendment on soil organic carbon storage under arid climate

Organic matter amendments have been proposed as a means to enhance soil carbon stocks on degraded soils, particularly under arid climate. Soil organic carbon (SOC) plays a critical role in terrestrial carbon cycling and is central to preserving soil quality. The effects of biowaste compost (BWC) on soil carbon storage were investigated. In addition, changes in soil organic matter (SOM) and even soil organic carbon (SOC) in BWC-amended soils following different applications were studied. The added BWC quantities were as followed: BWC/soil (weight/weight (w/w) respectively: 1/8, 1/4, and 1/2). The different BWC-amended soils were assessed during 180 days under arid ambient conditions and in comparison with control soil. Results showed a significant increase in SOM and SOC with relation to BWC quantities applied. This increase was relatively clear up to 120 days, after which decrease in SOM and SOC levels were observed. Furthermore, results showed improved microbiological activities of the amended soils in comparison with the control soil. This was reflected by the increase of the amended soils’ respirometric activities as cumulative carbon dioxide carbon (C-CO₂) as function of incubation time and also in terms of specific respiration expressed as C-CO₂/SOC ratios.

Implications: Mediterranean soils under arid climate such as Tunisian soils are poor in organic matter content. Biowastes are potential source for soil fertilization. Composting process is the best method for the stabilization of organic matter of diverse origins. The biowaste compost amendment improves the soil organic carbon storage and enhances the soil microbial activity.     

Coagulation of colloidal material in surface waters: the role of natural organic matter

Organic matter has a great influence over the fate of inorganic colloids in surface waters. The chemical nature and structure of natural organic matter (NOM) will be an important factor in determining whether colloids will be stabilised or destabilised by NOM. Under environmentally relevant conditions, the ubiquitous fulvic acids are likely to be responsible for coating and imparting a negative charge to colloids. If the adsorbed polyelectrolyte coating produces an increase in absolute surface potential, it will act to stabilise colloids in the water column. On the other hand, colloidal organic carbon, especially chain-like structures, has been shown to be involved in the aggregation of inorganic colloids through the formation of bridges. It is highly probable that both adsorption and bridging flocculation are occurring simultaneously in the natural aquatic environment. The importance of each process depends directly on the nature and concentration of organic matter in the system and indirectly on the productivity of the lake, its hydrological pathways, temporal variations, temperature, etc. The present paper reports such results and emphasises the need to discriminate the different kinds of NOM.     

Changes in soil organic carbon storage under grassland as evidenced by changes in sulphur input-output budgets

Information about temporal changes in soil organic carbon (C) pools may be obtained indirectly from changes in input-output budgets of organically combined nutrients such as sulphur (S). Sulphur budgets were therefore evaluated for Northern Ireland (NI) for the period 1940-1990, inclusive. These budgets indicated that the land or soil had acted first as a sink but then as a source for S, and that reserves of soil S built up between 1940 and 1965 were totally depleted by the mid-1980s. Pooled data from six long-term soil-monitoring sites on undisturbed grassland suggested that negative S budgets from the late-1970s onwards had been due to the net mineralization of soil organic matter and thus were indicative of net losses of organic C from surface soil horizons. There was some evidence that the decline in rainfall and fertiliser S inputs from the mid-1960s may have precipitated the breakdown of soil organic matter.     

Relating dissolved organic matter fluorescence and functional properties

The fluorescence excitation–emission matrix properties of 25 dissolved organic matter samples from three rivers and one lake are analysed. All sites are sampled in duplicate, and the 25 samples include ten taken from the lake site, and nine from one of the rivers, to cover variations in dissolved organic matter composition due to season and river flow. Fluorescence properties are compared to the functional properties of the dissolved organic matter; the functional assays provide quantitative information on photochemical fading, buffering capacity, copper binding, benzo[a]pyrene binding, hydrophilicity and adsorption to alumina. Optical (absorbance and fluorescence) characterization of the dissolved organic matter samples demonstrates that (1) peak C (excitation 300–350 nm; emission 400–460 nm) fluorescence emission wavelength; (2) the ratio of peak T (excitation 220–235 nm; emission 330–370 nm) to peak C fluorescence intensity; and (3) the peak C fluorescence intensity: absorbance at 340 nm ratio have strong correlations with many of the functional assays. Strongest correlations are with benzo[a]pyrene binding, alumina adsorption, hydrophilicity and buffering capacity, and in many cases linear regression equations with a correlation coefficient >0.8 are obtained. These optical properties are independent of freshwater dissolved organic carbon concentration (for concentrations <10 mg L⁻¹) and therefore hold the potential for laboratory, field and on-line monitoring and prediction of organic matter functional properties.     

Fluorescence characterization of cross flow ultrafiltration derived freshwater colloidal and dissolved organic matter

3-D fluorescence excitation-emission matrix (EEM) spectrophotometry was applied to investigate the fluorescence characterization of colloidal organic matter (COM) and truly dissolved organic matter (DOM) from an urban lake and a rural river fractionated by the cross flow ultrafiltration (CFUF) process with a 1kDa membrane. Relatively high tryptophan-like fluorescence intensity is found in the urban water, although the fluorescence of both water samples is mainly dominated by humic/fulvic-like fluorophores. During CFUF processing, the fluorescence intensities of humic/fulvic-like materials in the retentate increased rapidly, but a slight increase is also observed in the permeate fluorescence intensity. Very different ultrafiltration behaviour occurred with respect to the tryptophan-like fluorophore, where both permeate and retentate fluorescence intensities increase substantially at the beginning of the CFUF process, then tend to remain constant at high concentration factor (cf) values. Comparison with tryptophan standards demonstrates that freshwater tryptophan-like fluorescence is not dissolved and 'free', but is, in part, colloidal and related to the ultrafiltration behaviour of fulvic/humic-like matter. A good linear relationship between the retentate humic/fulvic-like fluorescence intensity and organic carbon concentration further reveals that fluorescent humic/fulvic-like substances are the dominant contributors to colloidal organic carbon, mainly in the colloidal fraction.     

Heavy metals and arsenic fixation into freshwater organic matter under Gammarus pulex L. influence

Organic sediments are a main sink for metal pollutants in aquatic systems. However, factors that make sediments a sink of metals and metalloids are still not clear. Consequently, we investigate the role of invertebrate shredders (Gammarus pulex L.) on quality of metal and arsenic fixation into organic partitions of sediment in the course of litter decay with laboratory microcosm experiments. During the decomposition of leaf litter, G. pulex significantly facilitated the development of small particles of organic matter. The capacity of metal fixation was significantly higher in smaller particles than leaf litter and litter residuals. Thus, G. pulex enhanced metal fixation into the organic partition of sediments by virtue of increasing the amount smaller particles in the aquatic system. Furthermore, invertebrates have a significant effect on formation of dissolved organic matter and remobilization of cobalt, molybdenum and cesium, but no significant effect on remobilization of all other measured elements.     

Effects on the function of Arctic ecosystems in the short- and long-term perspectives

Historically, the function of Arctic ecosystems in terms of cycles of nutrients and carbon has led to low levels of primary production and exchanges of energy, water and greenhouse gases have led to low local and regional cooling. Sequestration of carbon from atmospheric CO2, in extensive, cold organic soils and the high albedo from low, snow-covered vegetation have had impacts on regional climate. However, many aspects of the functioning of Arctic ecosystems are sensitive to changes in climate and its impacts on biodiversity. The current Arctic climate results in slow rates of organic matter decomposition. Arctic ecosystems therefore tend to accumulate organic matter and elements despite low inputs. As a result, soil-available elements like nitrogen and phosphorus are key limitations to increases in carbon fixation and further biomass and organic matter accumulation. Climate warming is expected to increase carbon and element turnover, particularly in soils, which may lead to initial losses of elements but eventual, slow recovery. Individual species and species diversity have clear impacts on element inputs and retention in Arctic ecosystems. Effects of increased CO2 and UV-B on whole ecosystems, on the other hand, are likely to be small although effects on plant tissue chemisty, decomposition and nitrogen fixation may become important in the long-term. Cycling of carbon in trace gas form is mainly as CO2 and CH4. Most carbon loss is in the form of CO2, produced by both plants and soil biota. Carbon emissions as methane from wet and moist tundra ecosystems are about 5% of emissions as CO2 and are responsive to warming in the absence of any other changes. Winter processes and vegetation type also affect CH4 emissions as well as exchanges of energy between biosphere and atmosphere. Arctic ecosystems exhibit the largest seasonal changes in energy exchange of any terrestrial ecosystem because of the large changes in albedo from late winter, when snow reflects most incoming radiation, to summer when the ecosystem absorbs most incoming radiation. Vegetation profoundly influences the water and energy exchange of Arctic ecosystems. Albedo during the period of snow cover declines from tundra to forest tundra to deciduous forest to evergreen forest. Shrubs and trees increase snow depth which in turn increases winter soil temperatures. Future changes in vegetation driven by climate change are therefore, very likely to profoundly alter regional climate.     

Relationship between (210)Pb(ex) activity and sedimentary organic carbon in sediments of 3 Chinese lakes

This report demonstrates that organic matter was an important factor in lake sediment ²¹⁰Pb_ex dating. Sediment cores from lakes in central and western China with different-trophic levels were collected, and the ²¹⁰Pb_ex activity and total organic carbon (TOC) were measured. The Rock-Eval pyrolysis technique was used to deconvolute TOC into free hydrocarbons (S1), thermally less-stable macromolecular organic matter (S2a), kerogen (S2b), and residual carbon (RC). The results show significant correlations between TOC and ²¹⁰Pb_ex, particularly between S2a and ²¹⁰Pb_ex, in all the sediment cores. This indicated that the algal-derived organic component S2a may play the most important role in controlling the distribution of ²¹⁰Pb_ex. Scavenging by algal-derived organic matter may be the main mechanism. As chronology is the key to the understanding of pollution reconstruction and early diagenesis in sediments, more attention should be paid to the influence of organic matter on ²¹⁰Pb_ex.     

Effects of organic matter content and composition on ammonium adsorption in lake sediments

The nature of the influence of organic matter (OM) on ammonium adsorption in lake sediments remains disputed. In this study, the kinetics and thermodynamics of ammonium adsorption were investigated on sediment samples with different OM contents (ignoring the effects of OM mineralization) previously collected from Lake Wuli, a northern bay of Lake Taihu, a shallow lake in southern China. The mechanisms of ammonium adsorption in these samples were characterized by Fourier transform infrared spectrometry and scanning electron microscopy. The results show that the ammonium adsorption capacity of the sediments is highly correlated with their OM content and with the humic content of the OM. The ammonium adsorption capacity of OM varies with its composition, i.e., with the surface properties of the different functional groups present. Indeed, humic acid was found to have a greater ammonium adsorption capacity by itself than when mixed with kerogen and black carbon, the mixture of the latter two components proving a better adsorbent than pure black carbon.     

Characteristics of organic precursors and their relationship with disinfection by-products

The molecular weight distribution and chemical composition of precursors and their relationship with disinfection by-products (DBPs) were investigated. Most of the organic matter responsible for the major DBP precursors in the Pan-Hsin water are small compounds with a molecular weight less than 1 kDa. The hydrophobic acids display the greatest ability to produce DBP. Therefore, effective removal of small molecules or hydrophobic acidic organics prior to disinfection process will significantly reduce the DBP concentration in the finished water. Although the coagulation process is effective in removing large organic precursors and the removal efficiencies of CHCl3 formation potential and organic carbon increase proportionally to the molecular weight of the precursors, the conventional treatment methods have limited efficiency in eliminating small precursors, which have high DBP formation potential.